# std::allocator

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< cpp‎ | memory

Defined in header `<memory>` |  |
template< class T > struct allocator; | (1) |
template<> struct allocator<void>; | (2) | (deprecated in C++17) (removed in C++20)

The `std::allocator` class template is the default Allocator used by all standard library containers if no user-specified allocator is provided. The default allocator is stateless, that is, all instances of the given allocator are interchangeable, compare equal and can deallocate memory allocated by any other instance of the same allocator type.

The explicit specialization for void lacks the member typedefs `reference`, `const_reference`, `size_type` and `difference_type`. This specialization declares no member functions. | (until C++20)

The default allocator satisfies allocator completeness requirements. | (since C++17)

### Member types

Type | Definition
`value_type` | `T`
`pointer` (deprecated in C++17)(removed in C++20) | `T*`
`const_pointer` (deprecated in C++17)(removed in C++20) | const T*
`reference` (deprecated in C++17)(removed in C++20) | `T&`
`const_reference` (deprecated in C++17)(removed in C++20) | const T&
`size_type` | std::size_t
`difference_type` | std::ptrdiff_t
`propagate_on_container_move_assignment` (C++11) | std::true_type
`rebind` (deprecated in C++17)(removed in C++20) | template< class U > struct rebind { typedef allocator<U> other; };
`is_always_equal` (C++11)(deprecated in C++23)(removed in C++26) | std::true_type

### Member functions

(constructor) | creates a new allocator instance (public member function)
(destructor) | destructs an allocator instance (public member function)
address (until C++20) | obtains the address of an object, even if operator& is overloaded (public member function)
allocate | allocates uninitialized storage (public member function)
allocate_at_least (C++23) | allocates uninitialized storage at least as large as requested size (public member function)
deallocate | deallocates storage (public member function)
max_size (until C++20) | returns the largest supported allocation size (public member function)
construct (until C++20) | constructs an object in allocated storage (public member function)
destroy (until C++20) | destructs an object in allocated storage (public member function)

### Non-member functions

operator==operator!= (removed in C++20) | compares two allocator instances (public member function)

### Notes

The member template class `rebind` provides a way to obtain an allocator for a different type. For example, std::list<T, A> allocates nodes of some internal type `Node<T>`, using the allocator `A::rebind<Node<T>>::other`(until C++11)std::allocator_traits<A>::rebind_alloc<Node<T>>, which is implemented in terms of `A::rebind<Node<T>>::other` if A is an `std::allocator`(since C++11).

Member type `is_always_equal` is deprecated via LWG issue 3170, because it makes custom allocators derived from `std::allocator` treated as always equal by default. std::allocator_traits<std::allocator<T>>::is_always_equal is not deprecated and its member constant `value` is true for any `T`.

### Example

Run this code

    #include <iostream>
    #include <memory>
    #include <string>

    int main()
    {
        // default allocator for ints
        std::allocator<int> alloc1;

        // demonstrating the few directly usable members
        static_assert(std::is_same_v<int, decltype(alloc1)::value_type>);
        int* p1 = alloc1.allocate(1); // space for one int
        alloc1.deallocate(p1, 1);     // and it is gone

        // Even those can be used through traits though, so no need
        using traits_t1 = std::allocator_traits<decltype(alloc1)>; // The matching trait
        p1 = traits_t1::allocate(alloc1, 1);
        traits_t1::construct(alloc1, p1, 7);  // construct the int
        std::cout << *p1 << '\n';
        traits_t1::deallocate(alloc1, p1, 1); // deallocate space for one int

        // default allocator for strings
        std::allocator<std::string> alloc2;
        // matching traits
        using traits_t2 = std::allocator_traits<decltype(alloc2)>;

        // Rebinding the allocator using the trait for strings gets the same type
        traits_t2::rebind_alloc<std::string> alloc_ = alloc2;

        std::string* p2 = traits_t2::allocate(alloc2, 2); // space for 2 strings

        traits_t2::construct(alloc2, p2, "foo");
        traits_t2::construct(alloc2, p2 + 1, "bar");

        std::cout << p2[0] << ' ' << p2[1] << '\n';

        traits_t2::destroy(alloc2, p2 + 1);
        traits_t2::destroy(alloc2, p2);
        traits_t2::deallocate(alloc2, p2, 2);
    }

Output:

    7
    foo bar

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2103 | C++11 | redundant comparison between `allocator` might be required | `propagate_on_container_move_assignment` provided
LWG 2108 | C++11 | there was no way to show `allocator` is stateless | `is_always_equal` provided

### See also

allocator_traits (C++11) | provides information about allocator types (class template)
scoped_allocator_adaptor (C++11) | implements multi-level allocator for multi-level containers (class template)
uses_allocator (C++11) | checks if the specified type supports uses-allocator construction (class template)

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